The present invention generally relates to multi-channel reproducing systems, and more particularly to a multi-channel reproducing system for reproducing audio signals in such a manner that a listener can listen to a reproduced sound with a presence which gives a feeling as if the listener is in a concert hall.
Generally, a listener in a concert hall listening to a music performed on a stage simultaneously hears a direct sound from a playing sound source and an indirect sound reflected from inner walls of the hall. The direct sound traveling directly to the listener from the sound source and the indirect sound traveling to the listener after being reflected by the inner walls have a difference in time for the sounds to reach the listener. Because of this arrival time difference .DELTA.T in the sounds reaching the listener, the listener can feel that he is listening to the musical performance in a concert hall.
Accordingly, there is a conventional four-channel reproducing system intended for reproducing audio signals picked up from a record disc or a tape so that the listener can hear the reproduced audio signal with a presence as if the listener is in a concert hall. In this system, two-channel stereo audio signals are reproduced and sounded from two main speakers as they are, while the aforementioned two-channel stereo audio signals are delayed for a predetermined time period by a digital delay circuit, for example, and delayed signals thus obtained are reproduced and sounded from two sub-speakers which are independent of the two main speakers.
However, this conventional four-channel reproducing system has a problem in that, although the system is capable of producing the presence more or less as compared to conventional simple two-channel reproducing systems, the sound in a high frequency range higher than about 4 kHz introduces an echo which is unnatural, and accordingly, the presence in the actual concert hall cannot be obtained.
The reason why this problem occurs was studied using recent reasearches and architectural acoustic data, which led to a finding that the arrival time difference .DELTA.T between the direct sound and the indirect sound does not only change with the distance from the sound source but also changes with the frequency of the audio signal. Thus, as the audio signal frequency becomes lower, the time difference .DELTA.T increases, and as the frequency becomes higher, the time difference .DELTA.T approaches a small constant value.
In the conventional system as described, the audio signal is delayed by the predetermined delay time uniformly throughout the whole frequency range irrespective of the frequency. Therefore, when the delay time of 100-200 msec is chosen in order to obtain a sufficient presence, the high frequency component having the delay time as described will result in an echo and causes a problem in that the presence becomes unnatural. On the other hand, when the delay time is shortened to 20 to 30 msec to eliminate the echo, a sufficient presence cannot be obtained.